Millwood is the name of a legendary forest, home to its own human population. The fact then that this woodland was “discovered” long after that community had left implies it to be located in a relatively remote part of the world, its people having no interaction with “civilized” society outside of tradition and hearsay. We cannot even say if Millwood culture used that name to describe themselves or the land they came from, since it literally refers to a forest with a mill — what better impetus for man suddenly discovering the wood than trying to expand and develop land for grain or lumber? Whatever the case, the people of Millwood seem to have settled such a hinterland because of their Ethereal Oak, or “spirit tree” (霊樹) to be more accurate. Unlike the Curse-rotted Greatwood, this tree uses the kanji for actual spirits rather than gods, implying Millwood religion to be more animist in nature. Indeed, the Japanese description for Earth Seeker notes it to be a “primitive earth faith”, with the symbol emblazoned on the worshipers’ equipment betraying the spirit tree’s central importance to it.
Battle axe used by a Millwood knight. Its blade is decorated with the symbol of the spirit tree.
Very heavy arms of robust warriors.
In that case, what made this tree so special? Its holy power. The Millwood tribe’s medium shields allegedly heal the wielder over time thanks to the Ethereal Oak’s “divine protection”, not necessarily because of the large symbol representing it on their face like the localization implies. Put simply, the tree itself possesses power akin to that of the Anor Londo gods, giving plenty of incentive for the Millwood community to gather around it; considering that descriptions for their armor describe it as “towering”, that tree had just as plenty of power to share. This notion is reinforced by the antlers of the spirit tree’s “holy beasts” (聖獣) seemingly belonging to deer. In Shinto, deer are considered divine messengers due to their habitation around certain holy sites, so we can infer that the area around the Ethereal Oak serves a similar function. In fact, the use of the term “holy beast” might even suggest a more intimate relationship between the two — deer are known to eat seeds and foliage, and such terminology has otherwise only been used for lightning-breathing chimera in the original Dark Souls.

Charbronze medium shield used by a Millwood knight. Very slowly recovers HP.
Is there still a little bit of the divine protection of the spirit tree it was greatly decorated with left?
Regardless of whether or not an entire species of deer evolved from feeding off of it, there is no doubt that the spirit tree was worth the worship it received. For that reason, certain individuals within the community were delegated to perform religious services not unlike Lothric’s High Priestesses, even using a similar term meaning “ritual chief”. (祭祀長) These high priests wield a ceremonial axe for their rituals, which apparently include praying to cause earthquakes. In fact, most Millwood weapon skills involve shaking or upheaving the earth in some fashion, with the Stonequake Hammer proving that this practice originates far back in the faith’s history. Where the deer holy beasts live in symbiosis with the spirit tree, the land actually nourishes it, prompting the worshipers to synergize with the earth. Even then, the development between the Quakestone Hammer and Earth Seeker highlights more similarities to Anor Londo religion. Not just in its “Earthen Wrath” skill’s similarity to “Wrath of the Gods”, but in Millwood’s association of bronze with holiness as well.
Large double-bladed axe made of bronze. It’s a ceremonial axe and originally used for religious rituals.
Millwood was a land of a primitive earth faith, and the knight chiefs were probably also the religious ritual chiefs.
Battle art is “Wrath of the Earth”. Violently thrust weapon into the earth alongside a prayer and cause a severe earthquake in the surrounding area. It is said that those tremors even resemble an explosion.
Rock hammer of Millwood knights. Natural rock has become its striking part as it is.
It is said to be an especially old weapon even in Millwood and was a symbol of battling alongside the earth.
Beyond all that, however, is the fact that these remote forest dwellers are wielding weapons despite the implicit lack of enemies to fight. Rather, this warrior culture seems to have sprung out of a simple desire to protect their spirit tree. Each Millwood “knight” decorates their helmets with the holy beasts’ antlers, implying that they think of themselves as comparable bearers of divine will; indeed, the high priests wear helms with even larger antlers and double as knight captains, reflecting different degrees of holiness between the tree’s supposed agents. This desire to serve and protect the tree explains why they are “knights”, embodying the chivalric ideal even absent any apparent monarchy or god. Naturally, this meant that they needed holy armor to match, fashioning themselves equipment made from charbronze — whereas bronze is literally “blue copper”, (青銅) charbronze is “burnt copper” (焦銅) charred bluish-black by intense heat. Their civilization’s entire identity revolved around the Ethereal Oak, which made it both the culture’s biggest strength and, ultimately, its Achilles’ heel.
Millwood’s discovery came with its own bizarre sight. Whatever buildings were constructed there were in ruins, the giant tree rotting. Something had clearly happened to their great earth spirit, but what? The obvious answer would be the only other detail of note regarding the Millwood knights’ history: the “abyss dragon” they so despised. And there is only one bit who matches this description: Midir. If the corrupted archdragon had become the knights’ bitter enemy, then he had most likely attacked the holy tree. As to why, we need only look at a few more peculiarities at the crime scene. The tree is rotted, not burned, and there apparently wasn’t a single corpse lying around the ruins. Surely, if Midir assailed Millwood in a random bout of lunacy, the entire forest would have been razed. Instead, the scoped was limited with practically surgical precision. And if he had killed the tree with claws instead of flames, then he seemingly did so without spreading his corruption to it.

Helmet of Millwood knights. It is decorated with antlers said to come from holy beasts of the spirit tree.
It is said that when the Millwood forest of legend was discovered, it was an uninhabited ruin where a rotted spirit tree towered. There wasn’t a single corpse there. Everything had been quietly abandoned.
Taken together, Midir’s motive was most likely not destruction, but self-preservation. By the nature of his mission and subsequent corruption, Midir must have known what afflicted him even as he continued to search the world for new Abysses to stamp out. Much like Artorias, the Dark would have slowly clawed at his mind, and he would have sought relief. In that case, how would it look when he came across a giant tree filled with divine power, the same power his masters so often used to exorcise the Dark he hunted? Simply put, Midir probably assaulted the spirit tree only out of a desperate desire to cleanse his corruption, much like how an animal might attempt to grind against tree bark to scratch an itch. With his mind already addled as it was, the best he could do was likely clumsily claw at it in hopes of better “immersing” himself in the tree’s power. It might not have even been just the one time.
The knights answered this attack with black oak greatbows and arrows, unconventional weaponry even for a culture favoring large hammers and axes. The design would imply that they expected to face something especially large for standard shooting, such as Midir. And given all the other parallels to Anor Londo, independently inventing the same ranged arms to combat archdragons is almost expected. With that in mind, Midir must have regularly visited the tree as he carried on his mission, giving time for Millwood’s resident knights to develop new arms to fend off the attacker — and building enmity towards their regular foe in the process. Perhaps this standoff began with Midir simply getting too close to the tree for some superficial contact, and only escalated as the dragon lost more and more of his mind. Whatever the case, he ultimately killed the tree along with any hope of finding relief. The death of such a powerful soul unsurprisingly had an adverse effect on the bits blessing the knights’ equipment, and whatever power remained in its bodily vessel would slowly rot away. The struggle was over.
Black oak greatbow of Millwood knights. Can only use greatarrows.
It is said that it was the arms for the knights of Millwood to answer the abyss dragon, who is their bitter enemy.
Although Midir presumably just moved on with his mission, Millwood was left to grapple with its failure to protect the great spirit. Considering that the culture essentially lost what tied the community together and gave their lives purpose, their inability to prevent it must have been devastating. In the end, they all chose to simply abandon everything save for the clothes on their backs and weapons they normally carried. This wasn’t to move on for themselves, however. Without the Ethereal Oak, there was no point in staying there, even if they had no place to go either. Bereft of meaning to their existence, they could only wander aimlessly as if in some form of purgatory. Eventually, the nature worshipers meandered into Ariandel — taking up residence in the painting world’s frigid woods where they have befriended the local wolves — but not without cost.

We neither see nor learn about women and children in Millwood despite their necessity for a remote, primitive culture. The knights apparently didn’t leave them behind when embarking upon their march, so why aren’t there any signs of them at the final destination? Moreover, all the Millwood knights we encounter are Hollows, having even forgotten about their nemesis during their long march to nowhere. This would imply that they died and turned Undead in the midst of their aimless wandering, their broken hearts ultimately fracturing their memories as well. Factoring in the fact that the community quietly abandoned their homes without taking provisions, and Millwood’s long march seems to have been a trail of tears, with who knows how many dying before the robust warriors we see remaining actually arrived as Hollows. They kept going in search of purpose while lacking direction absent one, leaving even each other behind along the way. Such is the tragedy of Millwood.
Ashen remains of a Millwood knight chief. The handmaid of the ritual place will have new items to offer.
In the remote past, the knights, who lost the forest that is their homeland, forgot even their bitter enemy in their long, aimless travels, discovering just a cold, withering forest at the end of it.